I purchased this LuniQuest softbox for my 580 EX II speedlite after I saw it recommended in 'Flash Techniques for Macro & Close-Up Photography' (by Deutschmann) - not a book I would particularly suggest by the way, there are much more inspirational and informative books on flash techniques to try - but being a novice when it comes to flash photography I was looking for some impartial advice. Other softboxes on Amazon had mixed reviews, so I decided to try the LumiQuest despite it being considerably more expensive than most of the others of equivalent size.
In terms of how it performs, it seems to do the job fine. You attach velcro strips to the speedlite, then the softbox can be attached and re-attached to these strips as and when it is needed. Once the softbox is attached it seems to stay on fairly firmly. The problem I had, however, was that the front of the diffuser panel tore within a week or two of me owning the product. I put this down to poor design. The softbox is a special shape - the front lower edge is a v-shape (as can be seen on the product photo) which is designed so that the pre-flash from the speedlite is not obstructed by the diffusing panel. The problem is this leads to weakness in edge of the diffuser - especially where the notch is cut into the side. The softbox is made from a sort of soft, flexible plastic, so it should be capable of a certain amount of flexing - especially given that the whole thing is designed to be taken on and off the speedlite regularly and to fold up into an envelope for ease of storage. Unfortunately the base of the softbox at the centre of the 'v' shape is just to weak and once its gone its not possible to repair even with tape (due to the uneven edge). I now have a softbox with a tear at the bottom which is getting larger every time I use the product. Shame really as in every other aspect it seems to work fine. I shall soon have to buy another one (after much less than a month of usage) and will not be purchasing from this manufacturer again.